1. Field of the Invention
The present invention relates to an electrode structure in a panel electrode etc. of a liquid crystal display unit, for example, to a part mounting structure in the panel electrode etc. and particularly to a liquid crystal display unit equipped with the part mounting structure.
2. Description of the Prior Art
Since a liquid crystal display unit, for example, is a planar display unit having excellent properties, such as small thickness, light weight, low power consumption, etc., it is put to a wide range of applications to mobile devices, such as a Personal Digital Assistance (so-called PDA) or a portable telephone, etc., to the display of a personal computer, etc.
The liquid crystal display unit has a liquid crystal panel of a structure having a liquid crystal layer sandwiched between a pair of display panel substrates, i.e. an array substrate and an opposed substrate, and controlled by applying voltage selectively to pixels between the array substrate and the opposed substrate to display images on the liquid crystal panel. In an active matrix liquid crystal display panel, for example, an amorphous silicon or polysilicon semiconductor is used for an array substrate to form a Thin-Film Transistor (TFT) as a switching device and simultaneously form pixel electrodes, scan lines, signal lines and so on connected to the switching device. On the other hand, an opposed substrate is formed with opposed electrodes of Indium Tin Oxide (ITO) or the like, color filters, etc.
In the liquid crystal display unit of the structure mentioned above, with an aim to reduce the weight and thickness thereof, the array substrate has a scan line-operating circuit and a signal line-operating circuit built-in. Particularly, since the signal line-operating circuit has to be operated at higher speed than the scan line-operating circuit, it is formed as an IC chip, for example, and the IC chip is mounted on the array substrate using the Chip On Glass (COG) technique. The IC chip is mounted as being connected onto a panel electrode provided on the outer edge of the array substrate via a bump, receives signals output from an external control circuit and inputs image-controlling signals to the scan lines and signal lines of the array substrate.
In case where the array substrate is formed with the panel electrode on which the IC chip is mounted, as described above, it is conceivable that it is advantageous to form the panel electrode in a multilayer structure for the purpose of securing the reliability thereof. To be specific, a conductor layer of ITO is stacked on a conductor layer of aluminum to protect the conductor layer of metal, thereby expecting suppression of corrosion occurrence etc. and improvement in the reliability.
Generally, a wiring layer formed on a substrate is covered with a solder resist layer to protect it from corrosion, damages and adverse affects by adherence of extraneous material. Furthermore, it is also discussed in JP-A 2000-75324 to form a protective layer on a wiring layer. Since the electrode portion requires electrical connection, it cannot be covered with the solder resist layer and is brought to an exposed state. While there is concern that the conductor layer in the electrode portion is corroded, therefore, it is conceivable to protect the conductor layer of metal from corrosion etc. when the metal conductor layer is covered with a layer of oxide, such as ITO.
In consequence of various studies conducted by the inventors, it has been found that a problem arises in terms of connection resistance where the conductor layer of aluminum has been covered with the conductor layer of ITO. Ordinarily, when the connection resistance of the IC chip mounted is thought of, the connection resistance between the bump and the electrode is mainly considered. When the panel electrode has been formed in the aforementioned multilayer structure, however, contact resistance will occur in a portion of contact between the conductor layer of aluminum and the conductor layer of ITO. The contact resistance is in inverse proportion to the area of contact between the conductor layer of aluminum and the conductor layer of ITO. In the liquid crystal display unit, since the COG portion has a smaller contact area than a Film On Glass (FOG) portion, for example, the COG portion is prone to high contact resistance and has a possibility of the electrical signals being adversely affected. Particularly, the input terminal portion is susceptible to this adverse affect.
The present invention has been proposed in view of the conventional state of affairs. One object of the invention is to provide an electrode structure and a part mounting structure capable of suppressing reliability degradation and connection resistance (contact resistance). Another object of the invention is to provide a part mounting structure and a liquid crystal display unit, with input signals not adversely affected, highly reliable in connecting parts.